The present invention relates generally to apparatus for heat-sealing thermoplastic sheeting and, more particularly, to such apparatus of the so-called L-sealer type adapted for use in packaging articles in heat-shrinkable plastic film.
In recent years, it has become increasingly popular and commonplace to package many commercial articles in close-fitting transparent plastic film. This packaging is accomplished conventionally by wrapping the article to be packaged loosely in a heat-shrinkable plastic film, sealing together the abutting edges of the film to enclose the article, and subjecting the package to sufficient heat to activate shrinkage of the film tightly about the article.
Machines commonly referred to as L-sealers are available for commercially accomplishing the preliminary wrapping procedure as above-described. Heat shrinkable plastic film in continuous sheets pre-folded along a central longitudinal edge and packaged in convenient rolled form is utilized for this purpose. L-sealing machines characteristically provide a sealing table for supporting the plastic film with the article inserted between the folds of the film and a pair of L-shaped jaws at the table adapted for receiving and grippingly engaging the film therebetween. One of the jaws is provided with a heated wire sealing arrangement to simultaneously sever and seal the film along a continuous L-shaped seal line extending parallel to the folded edge of the film and transversely between the parallel seal extent and the folded edge. In continuous operation, the transverse seal formed in making each package provides the initial transverse seal for the next package so that each described L-sealing operation provides complete package formation. Representative examples of L-sealing equipment of the described type are found U.S. Pat. Nos. 3,347,729; 3,490,981; 4,035,983; 4,650,535 and U.S. Pat. No. Re. 30,010. A more sophisticated L-sealing apparatus adapted for automatic high speed operation is disclosed in U.S. Pat. No. 4,219,988.
The operation of heating the package formed in the L-sealing operation is carried out in a machine commonly referred to as a shrink tunnel. Basically, such machines provide a substantially enclosed heating chamber through which travels a driven conveyor belt to transport the L-sealed packages through the heating chamber for shrinkage of the plastic film. Representative examples of shrink tunnels of this basic type are disclosed in U.S. Pat. Nos. 3,222,800; 3,312,811; and 3,316,653. Additionally, U.S. Pat. No. 3,200,561 discloses a similar plastic film heat-shrinking machine which does not utilize a substantially enclosed heat chamber but instead relies on a forced flow of heated air across the traveling conveyor.
As will be understood, the wrapping and sealing of an article within plastic film in the L-sealing operation serves to capture at least some quantity of air within the package enclosure defined by the sealed film. During the subsequent heat shrinking process, this captured air must be permitted to escape to prevent bursting of the film package. It is therefore conventional practice to provide a mechanism for punching holes at periodic intervals in the folded film as it is unwound from its roll during delivery to the L-sealer, the holes being formed at sufficiently close spacings along the length of the film to insure a minimum of one or more holes in each film package formed in the L-sealing operation. Typically, the holes are formed of a diameter of approximately one-quarter inch.
While this hole punching operation serves the intended purpose of permitting air escape during subsequent film shrinking, the holes in the film tend to permit entry of dirt and debris into the package which may soil the packaged article, thereby negating one of the principal objectives to be achieved by the overall film wrapping procedure. Invariably, a significant proportion of the packages produced have the holes located in the portions of the film covering the top and bottom surfaces of the enclosed article. Thus, dirt and debris on the conveyor of the heat shrinking machine may soil the article as it is subjected to the shrinking operation. Furthermore, the holes necessarily remain in the film of the finished package following the shrinking operation, the holes often being larger than originally formed, whereby an even greater risk exists of soiling by dirt and debris entering the package through the holes during subsequent handling of the finished package.